This invention relates generally to video display devices of the self-emitting type such as cathode ray tubes (CRTs) and is particularly directed to a method and apparatus for applying an antistatic/antireflective surface coating to the display screen of a CRT.
In a typical CRT, approximately 4-8% of the light incident on the CRT""s glass display screen is reflected. This reflected light not only degrades the resolution and contrast of the video image presented on the display screen, but also may cause eye fatigue and in some cases eye damage to the viewer. In addition, the high voltage, e.g., on the order of 25 KeV, typically generated in the CRT results in the buildup of electrostatic charge on the display screen. Dust tends to collect on the outer surface of the display screen because of the presence of this electrostatic charge, resulting in degradation of the video image. In addition, this electrostatic charge buildup may cause an electrical shock to the viewer, particularly in low relative humidity conditions.
In addressing the aforementioned problems, the outer surface of the CRT""s display screen is typically provided with a coating having antistatic, antiglare and antireflective characteristics. One approach to applying an antistatic/antireflective coating to the display screen""s outer surface employs a xe2x80x9cwetxe2x80x9d process known as spin or spray coating. Shown in FIG. 1 is a partial sectional view of a sealed glass envelope 10 of a CRT including a glass display screen 12 having disposed thereon a composite antistatic/antireflective coating 14 such as applied by one of the aforementioned wet coating processes. In FIG. 1 and the remaining figures shown and discussed below, common identifying numbers are used to identify the same element appearing in more than one figure. The composite antistatic/antireflective coating 14 applied to the outer surface of the glass display screen 12 is in the form of a single layer comprised of staggered molecules 16 dispersed within an insulating SiO2 layer 18. The inter-molecular spacing is relatively large and there is diffusion between the antistatic and antireflective layers so as to form a single conductive coating having a surface resistance on the order of 104-105 ohm/cm2. The composite antistatic/antireflective coating 14 is grounded by electrically coupling the antistatic/antireflective coating to the CRT""s grounded conductive implosion protection, or tension, band 20 by means of a conductive Al tape layer 22. A plastic film of a conductive Al foil 24 is then placed over the conductive Al tape layer 22 to provide physical protection and electrical insulation for the conductive Al tape layer. Because the composite antistatic/antireflective coating 14 is a good electrical conductor, electrostatic charge on the outer surface of the glass display screen 12 is effectively directed to neutral ground via the conductive Al tape layer 22 and the implosion protection band 20.
An antistatic/antireflective coating may also be applied to the glass display screen""s outer surface by a xe2x80x9cdryxe2x80x9d deposition process such as by sputtering. A sputtered antistatic/antireflective coating differs in several characteristics from an antistatic/antireflective coating deposited by spray or spin coating. For example, as shown in FIG. 2, an antistatic/antireflective coating 32 deposited by sputtering is comprised of an inner antistatic layer 26 and an outer antireflective layer 28. The reason for this difference is that in the sputter-deposited coating, the individual atoms are arranged in a staggered array, rather than the molecules as in the spray or spin-deposited coating, producing a far more compact structure. The inner antistatic layer 26 deposited directly on the outer surface of the glass display screen 22 is highly conductive, while the outer antireflective layer 28 is an insulator. Each of the layers in the sputter-deposited antistatic/antireflective coating 32 is clearly distinguishable from the other layer in the coating and the two layers have fundamentally different characteristics. Indium-doped tin oxide (ITO) is a typical composition for the inner conductive antistatic layer 26, while insulating SiO2 is a typical composition for the outer antireflective layer 28. A layer of Ti may also be disposed between the inner antistatic layer 26 and the outer antireflective layer 28. Attempting to use the grounding approach of FIG. 1 in the sputter-deposited, layered antistatic/antireflective coating 32 of FIG. 2 does not provide effective grounding for the CRT""s glass display screen 12. For example, placing a conductive adhesive layer 22 such as of Al tape in contact with and between the coating""s outer antireflective layer 28 and the CRT""s implosion protection band 20 does not provide an effective electrical conducting path for grounding electrostatic charge buildup on the display screen because the outer antireflective layer is not a good electrical conductor. Similarly, positioning a plastic film 24 such as of conductive Al foil 24 on the outer surface of the conductive adhesive layer 22 also does not provide an effective path to neutral ground for an electrostatic charge on the outer surface of the CRT""s glass display screen 12.
Referring to FIG. 3, there are shown another arrangement for directing an electrostatic charge on the display screen 12 of a sealed glass envelope 10 of a CRT to neutral ground. In the arrangement shown in FIG. 3, the layered antistatic/antireflective coating 32 is also formed by sputtering and is comprised of an inner antistatic layer 26 and an outer antireflective layer 28. A gap, or opening, has been formed in the outer antireflective layer 28 adjacent its peripheral edge and extends down to the conductive inner antistatic layer 26. A conductive element 42 is then inserted or formed in the opening in the outer antireflective layer 28 and is positioned in contact with the conductive inner antistatic layer 26. The conductive element 42 is typically comprised of a conductive metal and is formed by conventional means such as ultrasonic spot welding as in the xe2x80x9cSunbonderxe2x80x9d technique practiced by Asahi Glass Company of Japan. A conductive Al tape layer 44 is then positioned on the conductive element 42 and extends to the CRT""s implosion protection band 20. The conductive element 42 reduces the electrical resistance between the inner antistatic layer 26 and the conductive Al tape layer 44 to provide an effective electrical conductive path to neutral ground via the implosion protection band 20. A plastic film of conductive Al foil 46 is then applied over the conductive Al layer 44 as in the previously described other prior art approaches.
The conductive aluminum tape strip used in the prior art grounding arrangements described above and shown in FIGS. 1-3 is relatively expensive and thus increases the manufacturing cost of the CRT. In addition, the conductive aluminum tape strip is typically applied by hand by a worker which further increases CRT manufacturing costs. Finally, while incorporating a conductive element within the antistatic/antireflective coating by means of ultrasonic spot welding provides a good conductive path from the inner antistatic layer to neutral ground, this approach also increases the complexity of CRT manufacture resulting in a corresponding increase in manufacturing costs.
The present invention addresses the aforementioned limitations of the prior art by providing for the electrical grounding of an inner electrically conductive antistatic layer disposed on the outer surface of a CRT""s glass display screen where an outer non-conductive antireflective layer is disposed over the inner antistatic layer. The inner antistatic layer is first deposited on the entire outer surface of the display screen. A portion of the deposited antistatic layer is then masked prior to deposition of the outer antireflective layer leaving a portion of the inner antistatic layer exposed after the antireflective layer is applied. The exposed portion, or portions, of the antistatic layer is then electrically coupled to the CRT""s grounded implosion protection band for safely directing electrostatic charge on the display screen to neutral ground.
Accordingly, it is an object of the present invention to deposit an outer insulative antireflective layer on an inner conductive antistatic layer disposed on the outer surface of a CRT display screen, while leaving a portion of the inner antistatic layer exposed to facilitate grounding of the inner antistatic layer and discharge of electrostatic charge buildup on the display screen safely to neutral ground.
A further object of the present invention is to provide apparatus and a method for forming a two-layer coating comprised of a grounded conductive inner antistatic layer and an insulative outer antireflective layer on the outer surface of a video display screen by sputtering.
These objects of the present invention are achieved and the disadvantages of the prior art are overcome by a method and apparatus for applying an antistatic/antireflective coating to an outer surface of a glass display screen of a CRT, wherein a video image is produced on the glass display panel by plural electron beams incident upon a light emitting coating disposed on an inner surface of the glass display screen, and wherein an electrostatic charge is produced on the display screen by the electron beams incident thereon and incident light is reflected by the display screen, the method and apparatus comprising the steps of or means for applying a first electrically conductive antistatic layer by sputtering to the outer surface of the glass display screen; masking a peripheral portion of the first antistatic layer, and applying a second insulative antireflective layer on the first antistatic layer by sputtering, wherein the second antireflective layer is not disposed on the masked portion of the first antistatic layer leaving the masked portion of the first antistatic layer exposed for connection to an electrically grounded conductor for directing electrostatic charge on the display screen to neutral ground.